This chapter is from the book

This chapter is from the book

DSL and Cable Modem Networks

This chapter covers the following topics:

Digital Subscriber Line

Cable Access Technologies

DSL and cable modem network access are two alternative ways to connect to a
network service provider without the use of more expensive dedicated service,
such as Frac-T1/T1. DSL and cable modem networks achieve the same result of
providing dedicated access to a network service, often the Internet, but each do
so using differing technologies. This chapter discusses what DSL and cable modem
technologies do and how they do it.

Digital Subscriber Line

Digital subscriber line (DSL) technology is a modem technology using existing
twisted-pair telephone lines to carry high-bandwidth applications, such as
multimedia and video. The term xDSL covers a number of DSL technologies,
such as Asymmetrical Digital Subscriber Line (ADSL), Symmetrical Digital
Subscriber Line (SDSL), Hi-Speed Digital Subscriber Line (HDSL), HDSL-2
(HDSLv2), ITU DSL standard (G.SHDSL), ISDN Digital Subscriber Line (IDSL), and
Very-High-Data-Rate Digital Subscriber Line (VDSL).

xDSL services are dedicated point-to-point network access over twisted-pair
copper wire on the local loop (last mile) between a network service provider's
(NSP) central office (CO) and the customer site. xDSL also can be deployed
in intra-building and intra-campus environments, as illustrated in Figure
7-1.

xDSL offers two chief benefits over dial-up service:

Dial-up service is limited to 53.3 Kbps, whereas xDSL service can enable
up to 6.122 Mbps.

Dial-up service is initiated "on-demand" by the end-user, but
xDSL service is a dedicated connection, meaning that it is "always
on."

The following sections discuss ADSL. ADSL is often deployed in the small
office/home office (SOHO) environment and is the traditional DSL service for
residential deployment. The asymmetry is ideal in these environments because the
majority of upstream bandwidth is consumed by Internet requests; for example,
users navigating through web sites. These upstream requests are small compared
to the downstream response, such as the web site fulfilling the user's
request.

ADSL

ADSL technology makes more bandwidth available downstream, from a NSP central
office (CO) to the customer site, than it makes available upstream, from the
customer site to the CO. Figure
7-2 illustrates an example of an ADSL connection.

The asymmetry of ADSL, combined with always-on access (which eliminates call
setup), makes ADSL another solution for Internet/intranet surfing,
video-on-demand, and remote LAN access because users of these applications often
download more data than they upload.

ADSL Architecture

ADSL circuits connect ADSL modems on each end of a twisted-pair telephone
line, creating three data channels:

A high-speed downstream channelRanges from 1.5 to 9
Mbps.

A low-speed upstream channelRanges from 16 to 640
Kbps.

A basic telephone service channelThe basic telephone service
channel is split off from the digital modem by filters or plain old telephone
service (POTS) splitters, providing uninterrupted basic telephone service.

NOTE

The upstream and downstream bandwidth ranges depend upon the distance between
the customer site and the DSL provider's CO; the greater the distance, the
lower the bandwidth capacity.

Transport SystemProvides the carrier backbone transmission
interface for the DSLAM system. This device can provide service specific
interfaces such as T1/E1, T3/E3, OC-1/3, and STS-1/3.

Local Access NetworkUses the local carrier Inter-CO network
as a foundation, providing connectivity between multiple service providers and
multiple services users, often with Frame Relay or ATM switches.

Digital Subscriber Line Access Multiplexer
(DSLAM)Concentrates data traffic from multiple DSL loops onto the
backbone network for connection to the rest of the network.

DSL Transceiver Unit-Remote (xTU-R)The customer site
equipment for service connection to the DSL loop.

POTS SplittersOptional device at both CO and service user
locations, enabling the copper loop to be used for simultaneous DSL and
transmission and single line telephone service. POTS splitters come in two
configurations:

Single splitter version for mounting at the residence

Multiple splitter version for mass termination at the CO

POTS splitters are either passive or active. Active splitters require
an external power source, and passive splitters require no power and
often have a higher mean time between failure (MTBF) than the active splitter.
Passive splitters enable lifeline services, such as 911, in the event of a DSLAM
or xTU-R power loss; active splitters require backup power.

ADSL Data Rates

Downstream bandwidth depends on a number of factors:

Length of the copper line

Wire gauge of the copper line

Presence of bridged taps

Presence of cross-coupled interference

NOTE

Bridged taps are any cable pair spliced into the main pair. Many
unused bridged taps remain from the early days when party lines were the norm
and two or more taps were made on every line. Bridged taps cause undesirable
reflection that can distort the high-frequency signals in modern transmission
technologies.

Line attenuation increases with line length and frequency, and decreases as
wire diameter increases. Ignoring bridged taps, ADSL performs as shown in Table
7-1.

xDSL service will not work over fiber-to-the-curb (FTTC) implementations.
FTTC is the installation of optical fiber to within a thousand feet of
the home or office. Fiber-to-the-home (FTTH) is the installation of optical
fiber from the carrier directly into the home or office.

ADSL Standards and Associations

The American National Standards Institute (ANSI) Working Group T1E1.4
approved an ADSL standard at rates up to 6.1 Mbps (DMT/ANSI Standard T1.413).
The European Technical Standards Institute (ETSI) contributed an annex to T1.413
reflecting European requirements including a single terminal interface at the
premise side of the access circuit.

The ATM Forum and the Digital Audio-Visual Council (DAVIC) have both
recognized ADSL as a physical layer transmission protocol for unshielded twisted
pair (UTP) media.

NOTE

UTP is a popular type of cable consisting of two unshielded wires twisted
around each other. Because UTP cabling is cost efficient, it is used extensively
for local-area networks (LANs) and telephone connections. UTP cabling does not
offer the high bandwidth or protection from interference that is found with
coaxial or fiber optic cables; however, UTP is less expensive and easier to work
with than coaxial or fiber-optic.

Other xDSL Technologies

There are several xDSL implementations in addition to ADSL. These are as
follows:

Single-lined digital subscriber line (SDSL)A rate-adaptive
version of Hi-speed digital subscriber line (HDSL) which like HDSL is symmetric.
SDSL enables equal bandwidth downstream from a network service provider CO to
the customer site as upstream from the customer site to the CO. SDSL supports
data only (maximum of 1.544 Mbps) on a single line and does not support analog
calls.

High-data-rate digital subscriber line (HDSL)Developed by
Bellcore, high bit-rate DSL (HDSL)/T1/E1 technologies have been standardized by
ANSI in the United States and by ETSI in Europe. HDSL is a more cost-efficient
method of installing T1 service to a customer site than traditional
dedicated DS1 service.

HDSL 2Standard enabling symmetric service at T1 speeds using
a single-wire pair rather than the two pairs of HDSL service. HDSL-2 also was
developed as a standard by which different vendors' equipment can
interoperate.

Very-high-data-rate digital subscriber line
(VDSL)Transmits high-speed data over short reaches of twisted-pair
copper telephone lines, with a range of speeds depending on actual line length.
The maximum downstream rate under consideration is between 51 and 55 Mbps over
lines up to 1000 feet (300 m). Downstream speeds as low as 13 Mbps over lengths
beyond 4000 feet (1500 m) also are in consideration.